The present invention relates to gas-fired apparatus, and more particularly, to gas-fired heaters, in which combustion occurs within a sealed or substantially sealed combustion chamber. A gas flow device and method provide regulation of gaseous and/or particulate flows in respect to the combustion chamber. In an illustrated embodiment, the gas flow device provides in-take of ambient air for use as primary or secondary air during combustion with suppression of low frequency start-up vibrations and effective filtration of airborne particulate.
The present invention is particularly advantageous in connection with gas-fired heaters having a sealed combustion chamber and a burner that is supplied with a combustible fuel such as natural gas and combustion air from the surrounding environment. The invention is illustrated and described hereafter with respect to residential hot water heaters.
In such water heaters, the burner may comprise a porous surface burner such as an infrared burner or a blue flame burner. In the former, the primary air flow may exceed the stoichiometric ratio and no secondary air is required to complete combustion of the fuel. In the case of a blue flame burner, secondary air is generally required to complete the combustion process. In both cases, the primary or secondary air may be delivered to the burner after passing through an entrance opening in the sealed combustion chamber.
One problem encountered in connection with the operation of the foregoing heaters is the occurrence of low frequency resonant vibrations in the range of 1-100 hertz during combustion start-up. Such low frequency vibrations or noise may be suppressed by relieving pressure buildup in the combustion chamber. Such pressure relief may be provided by combustion chamber volume change or pressure relief as taught in U.S. Pat. No. 5,435,716 owned by the assignee of the present application.
As discussed in greater detail in U.S. Pat. No. 5,435,716, it is theorized that when a gas-fired heater is first placed into operation and the burner is lit there is an initial rapid expansion of the air/gas mixture in the combustion chamber. The expansion occurs so rapidly that the inertia of the column of air in the flue pipe or exhaust pipe open to the atmosphere is unable to accelerate fast enough to remove all of the expanding gases. As a result the pressure in the combustion chamber increases during a high pressure start-up cycle and restricts the flow of combustion air or an air/fuel mixture to the burner so as to inhibit combustion as well as the formation of combustion products and commence a low pressure start-up cycle. The increase in pressure in the combustion chamber may in fact cause the combustion air or air/fuel mixture to flow backwards. This causes the burner flame to greatly decrease in intensity and may even force the flame to extinguish. Once the hot combustion gases in the combustion chamber have had sufficient time to overcome the inertia of the gases in the flue pipe, they will have moved up the flue pipe, causing the low pressure cycle to exist in the combustion chamber. The low pressure cycle or vacuum created will rapidly draw in the combustion air or air/fuel mixture causing rapid expansion of the flame and the hot gases will result in a pressure increase, thereby starting the high pressure cycle over again.
The pressure relief device allows for the increase of pressure in the combustion chamber caused by the inertia of the air in the flue pipe or exhaust pipe to be relieved and not restrict the flow of the combustion air or air/fuel mixture, thereby greatly reducing or completely eliminating the noise that the hot water heater makes when engaged, and inhibiting non-uniform flow. Thereafter, there exists little fluctuation in the pressure in the combustion chamber and generally uniform combustion follows.
As described above the fluctuation in air pressure causes a low resonance frequency to be observed. The low resonance frequency is best characterized as a rumbling. Although over an extensive period of time the low resonance frequency will dissipate, most gas-fired heaters are not in continuous operation, and the resonance frequency occurs each time the burner is placed into operation.
Another problem often encountered in connection with the operation of the foregoing heaters is burner intake of flammable or ignitable vapor contained in the ambient air supply. The intake of such flammable vapor may result in an explosion and/or ignition of the vapor portions remote of the heater. Such explosion and/or remote ignition may be associated with a pilot light used in connection with the heater or the intermittent operation of the heater burner itself.
The vapor may result from any number of volatile liquid or gaseous sources such as gasoline, solvents, insecticides, propane and other such sources typically encountered at the sites of household heater applications. The obvious resolution of this problem is to pass the intake ambient air together with any flammable vapor contained therein through a flame trap or arrester. Such flame traps are well-known in the art and tend to contain combustion of the flammable vapor within the sealed combustion chamber.
However, the use of a flame trap is not entirety satisfactory since the ambient air also tends to contain particulate which may collect and block the passage of flammable vapor through the flame trap. Illustrative particulate includes lint from fabrics or the like, dust and other conventional solid contaminants found in household or residential ambient air. If particulate collection is sufficient to block gas flow through the flame trap, the burner combustion may be interrupted. On the other hand, if the particulate is combustible, it may serve as a wick or flame path to ignite flammable vapors outside the combustion chamber.
A related problem described in U.S. Pat. No. 5,317,992, which is also owned by the assignee herein, arises when it is necessary to achieve high burner loading in a relatively small space. In such cases, maintaining low NOx emissions becomes even more difficult as increased loading tends to increase the combustion temperature and carbon monoxide and NOx concentrations in the products of combustion. Substantially sealing the combustion chamber overcomes this tendency by causing a subatmospheric pressure condition in the combustion chamber sufficient to pull excess primary air through the burner to cool the flame and reduce the emissions of carbon monoxide and NOx to low levels. However, substantially sealing the combustion chamber also exacerbates the tendency of burners operating in combustion chambers to produce a resonance or combustion noise upon ignition of combustion. This resonance can persist for long times and can be unacceptably loud. The tighter the seal, the louder the noise, and the more difficult it becomes to control it.
It is the object of the present invention to overcome or minimize the disadvantages of the existing technology as described above. Such disadvantages specifically include the low resonance frequency noise observed when the gas-fired heater is initially placed into operation, the risk of explosion and/or ignition of flammable vapor that may be contained in ambient air and the avoidance of interference of burner operation by airborne particulate.
As indicated, the present invention contemplates a device and method for accommodating or suppressing pressure fluctuations within a sealed or substantially sealed enclosure or combustion chamber while concurrently providing for intake of combustion air. Further, the device and method may be arranged to provide regulation of the risks of particulate interference with burner operation and flammable vapor ignition.
Specifically, a gas-fired combustion apparatus or heater having a sealed or substantially sealed enclosure or combustion chamber includes a pressure relief device that operates to relieve or suppress pressure changes which may otherwise occur in the combustion chamber with commencement of the burner operation. To that end, ambient air enters the combustion chamber through a gas flow device arranged to pass ambient air for use in the combustion process and restrict particulate flow which may interfere with the combustion process and/or serve as a flame path to ignite flammable vapor remote of the combustion chamber. The particulate is concomitantly cleared from the gas flow device by the latter""s operation to relieve or suppress pressure fluctuations in the combustion chamber.
In an illustrated embodiment, the gas flow device comprises a flexible member extending across the entrance to the combustion chamber. The flexible member is gas permeable to allow passage of ambient air into the combustion chamber while restricting the entrance of particulate such as lint or dust. The flexible member is fixed about its periphery to the combustion chamber. The flexible member is sufficiently elastic to deform and oscillate or reciprocally move in the manner of a drum skin in response to pressure fluctuations. In this manner, the flexible member provides both volume changes and venting of gas from the interior of the combustion chamber in order to reduce the low frequency vibrations.
The pulsing oscillations or reciprocal movement of the flexible member tends to dislodge particulate collected on the surface of the flexible member remote of the combustion chamber. That is, the particulate falls away from the surface of the flexible member and back into the environment.
The flexible member may be formed of any suitable extensible material. The extensible material may be formed of polymers, woven or nonwoven fibrous webs and combinations thereof. As used herein, extensible contemplates a degree of elastic deformation sufficient to accommodate the oscillations of the flexible member.
If the material does not have sufficient permeability for the desired gas flow, openings may be provided in the member. Illustrated materials include synthetic and natural rubbers, silicone polymers, urethane polymers and fluoride polymers as are well-known in the art.
It should be appreciated that the overall size, thickness and degree of permeability or opening sizes of the flexible member together with the modulus properties of the material forming the member cooperatively provide sufficient movement of the member in response to pressure fluctuations to achieve the desired pressure relief. In addition to pressure relief, sufficient movement of the flexible member is provided to shake free particulate collected on the surface of the member remote of the combustion chamber.
The foregoing regulation of particulate permits the safe use of a flame trap located downstream of the flexible member to receive the incoming ambient air. The incoming ambient air flows through the flame trap in order for the latter to quench flame propagation to the exterior of the combustion chamber. Suitable flame traps or flame arresters are well-known in the art and typically include a heat resistant permeable material.
Flame traps typically include one or more woven or mesh screens formed of metal. Suitable metals include woven stainless-steel, inconel 601 wire mesh or the like. The flame trap may also be formed of a porous ceramic element such as a SCHWANK type ceramic tile.